To measure the coefficient of performance of a mechanical vapor-compression refrigeration system, two parameters must be known. These parameters are the input power, and the heat rejected (in the case of heating) or the heat absorbed (in the case of cooling). Currently, manufacturers test mechanical vapor-compression systems by measuring input power directly with a power transducer or indirectly with an ammeter, and by measuring the heat rejected or absorbed with a larger environmental chamber. This method allows accurate coefficient of performance measurement; however, it does not lend itself to application in the field, whether in the research laboratory or at a specific site, such as a home in which a heat pump or air conditioning system of the vapor-compression type is installed.
The need to measure the coefficient of performance of an air conditioning system or a heat pump has been recognized and is becoming more and more important with the widespread use of heat pumps as an energy efficient approach to heating and cooling. Attempts to provide measurements of the coefficient of performance have been undertaken by measurements of certain temperatures at a specific installation, but these measurements allow for only a relative coefficient of performance to be provided. The relative coefficient of performance provides a measure only of whether the efficiency of the particular vapor-compression system is increasing or is decreasing, but is incapable of delivering a specific coefficient of performance for the system.